Long-tailed JFET pair question

Whoops, right. I knew that.

With a 300 kelvin load resistor, dropping 200 mV gets you to 1 dB above the shot noise--and it's the AC resistance times the DC photocurrent that's relevant, if you're using a transformer. Above there, the NF is pretty well inversely proportional to voltage drop--400 mV gets you 0.5 dB, and so on.

Cheers,

Phil Hobbs

On Mon, 27 Nov 2006 17:58:05 GMT, Joerg wrote in Msg.

There is no following TIA. The drains go straight into the +/- inputs of the following opamp. Feedback is via high-value resistor back to the summing junction (the non-grounded FET gate), so the whole thing is a TIA.

robert

On Mon, 27 Nov 2006 18:44:21 +0100, Fred Bartoli wrote in Msg.

You confirmed analytically which had been my hunch supported by some wild guessing. For the OpAmp following the JFETs I had thought about using an LT1097 which isn't so great noise-wise but can be overcompensated externally in case my combined loop gain causes stability trouble.

robert

I used to work for a Co doing discrete opamps, (mumble, mumble) years ago. The jfet opamps were not my designs, but afair the drain loads were always pnp current mirrors of some sort.

--
Tony Williams.

Hello Phil,

The only fly in the ointment is probably going to be the DFB module. The datasheets on those are pretty skimpy. Beside the RIN all they tell you is that the line width is 10MHz max. That's a whole lot and who knows what kind of noise is in there. AM noise can be handled, if necessary with a differential amp where the MPD gets fed into the other input. But that won't help with phase noise :-(

--
Regards, Joerg

http://www.analogconsultants.com

The Sirenza SiGe parts are very quiet. One of them actually has a

50-ohm input impedance, a rarity among 50-ohm mmics. The MiniCircuits ERA-series tends to run in the high 30's. Depends on output loading and operating current, of course.

John

I've been a temp, on almost exactly the same type of work, except they found out I was a tech, and started using me for tech work, at temp pay. (well, $8.00/hr is infinitely better than $0.00/hr., so I shouldn't snivel too much. :-) )

Tell the agency exactly what you've said here, and someone will show up.

And you really don't need to know any electronics to do teensy soldering work - all you need is good eyes and a steady hand. :-)

Good Luck! Rich

Thing is the temp agencies usually don't understand. They don't know what 0402 really means. I've had folks who were claimed to be able to handle "fine stuff" and it didn't work at all.

Err, it does require a basic understanding of ESD. IOW some training.

Thanks, we'll need it.

--
Regards, Joerg

http://www.analogconsultants.com

Too bad that you're on the other side of the country, and that my eyesight is getting so bad. I did a lot of SMD rework under a stereo microscope. I routinely worked with 1206, 0805 and 0402 components, along with ICs and other SMD components.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Phase noise nearly always dominates the line width of lasers, because the total pump current sets an upper limit on the output power. I gather you're building an interferometer, otherwise the frequency noise isn't a problem.

Cheers,

Phil Hobbs

Something like that, and phase noise is going to be critical. My gut feeling is that it will be the limiting factor here. It'll be interesting.

The strangest thing for an EE to work with optical engineers is that they think in wavelengths, we think in frequencies. Almost like driving in England or Ireland. Had a PID regulator backwards this morning but caught it.

--
Regards, Joerg

http://www.analogconsultants.com

Physicists think in frequency, but use the opposite Fourier transform sign convention. It's almost as bad.

Diode lasers are also exquisitely sensitive to back-reflections--those noise specs probably assume a test setup with a two-stage Faraday isolator. Anything worse than about a 60 dB return loss will make that puppy howl.

One good thing about DFBs is that you can FM them very well and very fast, so if you can arrange the measurement so that the signal comes in offset in frequency, you can filter out most of the junk. What remains is multiplicative phase noise. Assuming the signal large compared with the shot noise, the SNR will be constant with signal level, rather than disappearing into the grass.

A typical method for doing this is sinusoidal FM, such that (Delta f)(t_path_difference) = 2.405 radians, the first Bessel null. About there, the first and second harmonics are equal in size and together amount to about 85% of the total electrical power.

Cheers,

Phil Hobbs

Yes, that issue has me worried a bit. Also, I have read that pigtailed versions make life a bit easier for reflections.

Thanks, Phil. I will keep that in mind. Right now it's all new and I am not sure whether we can do FM, considering the signal BW of up to around

100MHz. But it would be no problem to provide a really wide bandwidth receiver so we can play some nifty filtering/demodulating tricks afterwards. At least then I'd be back in the RF world, my home turf ;-)

--
Regards, Joerg

http://www.analogconsultants.com